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R-242M - 5

1.  When the oven door is opened, the oven lamp comes on

at this time.

2. When the cooking time is up, a single tone is heard and

the relays RY1 + RY2 go back to their home position.
The circuits to the oven lamp, power transformer, fan
motor and turntable motor are cut off.

CONNECTED COMPONENTS

RELAY

Oven lamp, Fan motor, Turntable motor

RY1

Power transformer

RY2

CONDITION

DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

Monitor switch

COM-NC

Open

Closed

Monitored latch switch

COM-NO

Closed

Open

Stop switch

COM-NO

Closed

Open

3. When the door is opened during a cook cycle, the

switches come to the following condition.

The circuits to the power transformer, fan motor and
turntable motor are cut off when the monitored latch switch
and stop switch are made open.
The oven lamp remains on even if the oven door is opened
after the cooking cycle has been interrupted, because the
relay RY1 stays closed. Shown in the display is the remaining
time.

When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the power transformer
intermittently within a 32-second time base through the
relay contact which is coupled with the current-limiting relay
(RY2). The following levels of microwave power are given.
SETTING

MEDIUM HIGH

MEDIUM LOW

Approx. 10% = 80 Watts

32 sec. ON

6 sec. ON

12 sec. ON

18 sec. ON

24 sec. ON

26 sec. OFF

20 sec. OFF

14 sec. OFF

8 sec. OFF

LOW

MEDIUM

HIGH

Approx. 70% = 560Watts

Approx. 50% = 400 Watts

Approx. 30% = 240 Watts

NOTE: The ON/OFF time ratio does not exactly correspond

to the percentage of microwave power, because
approx. 3 seconds are needed for heating up the
magnetron filament.

Approx. 100% = 800 Watts

The door can be opened by pulling the door.

1. When the oven door is closed, the contacts (COM-NO)

must be closed.

1. When the oven door is closed, the contacts (COM-NC)

must be opened.

2. When the oven door is opened, the contacts (COM-

NC) must be closed.

3. If the oven door is opened and he contacts (COM-NO)

of the monitored latch switch fail to open, the fuse F1
blows immediately after closing the contacts (COM-
NC) of the monitor switch.

CAUTION: BEFORE REPLACING A BLOWN FUSE  F1

TEST MONITORED LATCH SWITCH (SW1)
AND  MONITOR SWITCH FOR PROPER
OPERATION.  (REFER TO CHAPTER “TEST
PROCEDURE”).

1. The fuse F1 blows when the contacts (COM-NO) of the

monitored latch switch remain closed with the oven
door open and when the contacts (COM-NC) of the
monitor switch are closes.

2. If the wire harness or electrical components are short-

circuited, the fuse F1 blows to prevent an electric shock
or fire hazard.

The thermostat protects the high voltage transformer
against overheating. If the temperature goes up higher
than 150˚C because the fan motor is interrupted or the
ventilation openings are blocked, the thermostat will cycle,
line voltage to the high voltage transformer will also cycle.
(If operated, check the magnetron for damage.)

Latch hook

SW3: Stop switch

Door

SW2: Monitor switch

SW1: Monitored latch switch

Latch heads

6

R-242M -

MICROWAVE OUTPUT POWER (IEC-705)

The power output of this oven is rated using the method specified by IEC-705.  Full details of how to carry out this
procedure can be found in the Sharp Technical Training notes which is available from Sharp Parts Centre
(part number SERV-LITMW01).

Using this procedure, the heating time to raise 1000g of water by 10

°

C is approximately 50 seconds.

The IEC-705 procedure must be carried out using laboratory-type procedures and equipment.  These requirements
make the procedure unsuitable for routing performance checks.  An indication of the power being produced by the oven
can however be obtained using the procedure given below.

Alternative simplified method:
1. Place 2 litres of cold water (between 12

°

C and 20

°

C) in a suitable container.

2. Stir the water and measure the temperature in 

°

C.  Note temperature as T1.

3. Place the container in the microwave and heat the water for 2 minutes on full power.
4. When the 2 minutes is completed, remove the container and stir the water.  Note the water temperature as T2.
5. Calculate the output power using the following formula:

R.F. Power Output = (T2 - T1) x 70.

MICROWAVE LEAKAGE TEST

This oven should be tested for microwave leakage on completion of any repair or adjustment, following the procedure
described in the Sharp Technical Training notes (part number SERV-LITMW01).  The maximum leakage permitted in
BS EN 60335-2-25 is 50W/m

2 

(equivalent to 5mW/cm

2

), however it is not normal to detect any significant leakage,

therefore, any leakage which is detected should be investigated.

It is essential that only leakage detectors with current calibration traceable to the National Physical Laboratories are used.

Suitable leakage detectors :

CELTEC A100
APOLLO X1

The thermal cut-out located on the top of the oven cavity is
designed to prevent damage to the oven, if the food in the
oven catch fire due to over heating produced by improper
setting of cook time or failure of control unit. Under normal
operation, the thermal cut-out remains closed. However,
when abnormally high temperatures are reached within the
oven cavity, the thermal cut-out will open at 125˚C, causing
the oven to shut down. The defective thermal cut-out must be
replaced with a new one.

The turntable motor drives the turntable roller assembly to
rotate the turntable.

The fan motor drives a blade which draws external cool air.
This cool air is directed through the air vanes surrounding
the magnetron and cools the magnetron. This air is chan-
nelled through the oven cavity to remove steam and vapours
given off from heating food. It is then exhausted through the
exhaust vents at the rear of the oven cavity.

The noise filter assembly prevents radio frequency interfer-
ence that might flow back into the power circuit.

R-242M - 7

The touch control panel consists of circuits including semiconductors such as LSI, ICs, etc. Therefore,
unlike conventional microwave ovens, proper maintenance can not be performed with only a voltmeter
and ohmmeter.
In this service manual, the touch control panel assembly is in one unit and troubleshooting by unit
replacement is described according to the symptoms indicated.
Control Panel.

The following symptoms indicate a defective control unit.

1.  Tact Switch.

The following symptoms indicate a defective tact switch. Replace the tact switch.

a) When touching a tact switch, a certain tact switch produces no signal at all.
b) When touching a tact switch, sometimes a tact switch produces no signal.

2. In connection with tact switches.

a) When touching a tact switch, a certain group of tact switch do not produce a signal.
b) When touching a tact switch, no tact switch produce a signal.

3. Display problems.

a) At a certain digit, all or some segments do not light up.
b) At a certain digit, brightness is low.
c) Only one indicator does not light up.
d) The corresponding segments of all digits do not light up; or they continue to light up.
e) Wrong figure appears.
f) A certain group of indicators do not light up.
g) The figure of all digits flicker.
h)  When touching a tact switch, the control unit does not respond.

4. Other possible problems caused by defective control unit.

a) Buzzer does not sound or continues to sound.
b) Cooking is not possible.

 To prevent an electical shock, do not touch the printed wiring board or electrical parts.
Because the control unit is " TRANSLESS CIRCUIT " and all electrical parts
are used at A.C. line voltage.

1. Disconnect the oven from the power supply.
2. Discharge the high voltage capacitor.
3. Remove the control unit from the control panel.
4. By using an ohmmeter, check the tact switch operation.
5. When the tact switch is not depressed, an ohmmeter should indicate an open circuit. When the tact

switch is depressed, an ohmmeter should indicate a short circuit. If improper operation is indicated,
the tact switch is probably defective and should be checked.

Remove the outer case and check voltage between Pin No 5 of the 3 pin connector (A) and common
terminal of the relay (RY2) on the control unit with an A.C. voltmeter.
The meter should indicate rated voltage, if not check oven circuit.

RY1 and RY2 Relay Test
These relays are operated by D.C. voltage
Check voltage at the relay coil with a D.C. voltmeter during the microwave cooking operation.

DC. voltage indicated ............. Defective relay.
DC. voltage not indicated ....... Check diode which is connected to the relay coil. If diode is good,

control unit is defective.

RELAY SYMBOL

OPERATIONAL VOLTAGE

CONNECTED COMPONENTS

RY1

Approx. 19.0V D.C.

Oven lamp / Turntable motor / Cooling fan motor

RY2

Approx.   7.5V D.C.

Power transformer

8

R-242M -

This circuit generates voltage necessary in the control
unit from the AC line voltage.

Symbol

Voltage

Application

VC

+5V

LSI(IC1)

To drive the magnetron, fan motor, turntable motor and
light the oven lamp.

The buzzer is responsive to signals from the LSI to emit
audible sounds (tact switch sound and completion
sound).

The power source synchronizing signal is available in
order to compose a basic standard time in the clock
circuit. It accompanies a very small error because it
works on commercial frequency.

A switch to "tell" the LSI if the door is open or closed.

A circuit to drive the back light (Light emitting diodes
LED1-LED3)

The touch control section consists of the following circuits
as shown in the touch control panel circuit.

The principal functions of these circuits and their related
signals are explained below.

Signals generated in the LSI are sent to the tact switches
through R60, R61, R62 and R63.
When a tact switch is touched, a signal is completed
through the tact switch and passed back to the LSI through
R81 to perform the function that was requested.

Control unit consists of LSI, ACL circuit, indicator circuit,
power source circuit, relay circuit, buzzer circuit, synchro-
nizing signal circuit and back light circuit.

This circuit generates a signal which resets the LSI to
the initial state when power is supplied.

This circuit consists of 4-digits, 12-segments and 3-
common electrodes using a Liquid Crystal Display.

OUTLINE OF TOUCH CONTROL PANEL

NOTE: *At the time of making these repairs,

make a visual inspection of the varistor.
Check for burned damage. If any ab-
normal condition is detected, replace
the defective parts.

1

5

RY2

(J1)

CN-A 

RY1

D1

VRS1

D71

ZD71

To protect the electronic circuits, this model is provided with a fine foil pattern added to the input circuit
on the PWB, this foil pattern acts as a fuse. If the foil pattern is open, follow the troubleshooting guide
given below for repair.
Problem: POWER ON, indicator does not light up.

STEPS

OCCURRENCE

CAUSE OR CORRECTION

1

The rated voltage is not applied between Pin

Check supply voltage and oven power cord.

No. 5 of the 3 pin connector (A) and the

common terminal of the relay RY2.

2

Only pattern at "a" is broken.

*Insert jumper wire J1 and solder.

3

Pattern at "a" and "b" are broken.

*Insert the coil RCILF2003YAZZ between "c" and "d".